An intricate pattern of interactions within and between cells directs the sequential development of neurons from dividing neuroepithelial progenitor cells. Multiple extracellular and intracellular signals moderate this process. Among the key intracellular signals are transcription factors, which induce the expression of a cascade of genes. One subclass of transcription factors, belonging to the basic helix-loop-helix (bHLH) family of proteins, is expressed early on when the decision to proliferate or differentiate is made. This function is a particularly crucial one as mutations in these genes early in development can wipe out entire neural structures.
In Drosophila, the gene atonal (ato), which is homologous to Math1, Math2, Hath1 and Hath2, encodes a bHLH protein essential for the development of chordotonal organs (sensory organs found in the body wall, joints and antenna that function in proprioception, balance and audition) (Eberl, 1999; McIver, 1985; van Staaden and Römer, 1998). CHOs populate the peripheral nervous system (PNS) in the body wall and joints (thorax, abdomen, sternum, wings, legs) and antennae (Moulins, 1976), providing the fly with sensory information much as touch and mechanoreceptors do in vertebrates (McIver, 1985; Moulins, 1976). Boyan (Boyan, 1993) proposed that, in the course of evolution, different CHOs became specialized for hearing in different insects. This hypothesis was recently confirmed by van Staaden and Romer (1998). In Drosophila, CHOs in the Johnston organ, located in the second antennal segment, function in near field hearing (Dreller and Kirschner, 1993; Eberl, 1999) and negative geotaxis.
During development ato is expressed in a cluster of progenitor cells from which the CHO founder cells are selected (Jarman et al., 1993). It likely functions by regulating the expression of genes necessary for the specification and development of the CHO lineage; as it encodes a basic helix-loop-helix protein (bHLH) that dimerizes with the Daughterless protein and binds to E-box sequences, thereby activating genes (Jarman et al., 1993). CHO specificity is encoded by the ato basic domain, which is required for DNA binding in bHLH proteins (Chien et al., 1996; Davis et al., 1990; Jarman and Ahmed, 1998; Vaessin et al., 1990). ato is both necessary and sufficient for the generation of CHOs in the fly: loss of ato function leads to the loss of CHOs, while ectopic ato expression causes ectopic CHO formation (Jarman et al., 1993). Adult flies that lack atonal function are uncoordinated, do not fly, and are deficient in hearing. Overexpression of the fly atonal gene can generate new chordotonal neurons, indicating that atonal is both essential and sufficient for the development of this neuronal population.
In vertebrates, during myogenesis and neurogenesis, cell fate specification requires basis helix-loop-helix (bHLH) transcription factors. Math1 (for mouse atonal homolog-1) is such a factor, and is expressed in the hindbrain, dorsal spinal cord, external germinal layer of the cerebellum, gut, joints, ear and Merkel cells of the skin (which function as mechanoreceptors) (Akazawa et al., 1995; Ben-Arie et al., 1996; Ben-Arie et al., 1997). Mice heterozygous for a targeted deletion of Math1 (Math1+/−) are viable and appear normal, but Math1 null mice (Math−/−) die shortly after birth and lack cerebellar granule neurons.
Math1 is one of ato's closest known homologs, with 82% amino acid similarity in the bHLH domain and 100% conservation of the basic domain that determines target recognition specificity (Ben-Arie et al., 1996; Chien et al., 1996). Math1 is transiently expressed in the CNS starting at embryonic day 9 (E9) in the dorsal portion of the neural tube. Math1 is also expressed in the rhombic lip of the fourth ventricle of the brain, where cerebellar granule cell precursors are born at E13–15 (Alder et al., 1996). Upon proliferation and differentiation, these progenitor cells migrate to form the external granule layer (EGL) of the cerebellar primordia (Hatten and Heintz, 1995). Proliferating EGL cells continue to express Math1 during the first three postnatal weeks, until shortly before they migrate to their final adult destination to generate the internal granule layer (IGL) of the cerebellum (Akazawa et al., 1995; Ben-Arie et al., 1996). Another group of cells, a small population of neuronal precursors in the dorsal spinal cord, expresses Math1 during E10–E14 (Akazawa et al., 1995; Ben-Arie et al., 1996). These precursor cells also express the LIM homeodomain proteins (LH2A and LH2B), markers of the D1 class of commissural interneurons (Lee et al., 1998). Helms and Johnson (1998) reported that lacZ expression under the control of Math1 regulatory elements reproduced Math1 expression patterns in the developing cerebellum and spinal cord, and demonstrated that Math1 is expressed in precursors that give rise to a subpopulation of dorsal commissural interneurons.
To determine the in vivo function of Math1, the inventors generated mice (Math1−/−) lacking the MATH1 protein. This null mutation causes major cerebellar abnormalities: lack of granule cell proliferation and migration from the rhombic lip at E14.5, and absence of the entire EGL at birth (Ben-Arie et al., 1997). It is not clear whether the agenesis of cerebellar granule neurons is due to failure of progenitor specification or the cells' inability to proliferate and/or differentiate. Neonates cannot breathe and die shortly after birth, but there are no gross defects in any cranial nerves or brain stem nuclei that could explain respiratory failure.
The fact that Math1 is expressed in the inner ear suggests that Math1 expression is necessary for the development of auditory or balance organs. The inner ear initially forms as a thickening of the ectoderm, termed the otic placode, between rhombomeres 5 and 6 in the hindbrain. The otic placode gives rise to neurons of the VIIIth cranial nerve and invaginates to become the otocyst, from which the inner ear will develop. The mature mammalian inner ear comprises one auditory organ, the cochlea, and five vestibular organs: the utricle, the saccule, and three semicircular canals. The sensory epithelia of these organs consist of mechanoreceptive hair cells, supporting cells and nerve endings. Hair cells serve as mechanoreceptors for transducing sound waves and head motion into auditory and positional information. Hair cells and supporting cells both arise from a common progenitor cell and proliferate and differentiate within the sensory epithelia, with peak mitoses between embryonic day 13 and 18 (E13–18) in mice. Although several genes have been implicated in the development of the inner ear, such as int2 (Mansour et al., 1993; pax2 (Torres et al., 1996; and Hmx3 (Wang et al., 1998). None have been shown to be required for the genesis of hair cell specifically.
Damage to hair cells is a common cause of deafness and vestibular dysfunction, which are themselves prevalent diseases. Over 28 million Americans have impaired hearing; vestibular disorders affect about one-quarter of the general population, and half of our elderly. The delicate hair cells are vulnerable to disease, aging, and environmental trauma (i.e., antibiotics, toxins, persistent loud noise). Once these cells are destroyed, they cannot regenerate in mammals. Therefore, a need exists to address the problems of patients with congenital, chronic or acquired degenerative hearing impairment and loss or balance problems, and to provide compositions, methods and reagents for use in treating hearing loss and vestibular function.
In support of the teaching of the present invention, others have demonstrated that Math1, upon overexpression, induces significant production of extra hair cells in postnatal rat inner ears (Zheng and Gao, 2000). Briefly, although fate determination is usually completed by birth for mammalian cochlear hair cells, overexpression of Math1 in postnatal rat cochlear explant cultures results in additional ear hair cells which derive from columnar epithelial cells located outside the sensory epithelium in the greater epithelial ridge. Furthermore, conversion of postnatal utricular supporting cells into hair cells is facilitated by Math1 expression. The ability of Math1 to permit production of hair cells in the ear is strong evidence in support of the claimed invention.